This invention pertains to video driver circuits and more particularly to a wideband class AB driver circuit suitable for receiving a video input signal and, in response thereto, driving a video display.
Prior art class A wide bandwidth amplifiers have been used to amplify video signals, with three such circuits required for color systems, one for each of the three primary colors red, green, and blue. Such prior art class A wide-band amplifiers are described, for example, in "A New Wide-Band Amplifier Technique", Barrie Gilbert, IEEE Journal of Solid-State Circuits, Vol. SC-3, pages 353-365, December 1968; "Design Techniques for Monolithic Operational Amplifiers", Robert J. Widlar, IEEE Journal of Solid-State Circuits, Vol. SC-4, No. 4, page 184-191, August 1969; and U.S. Pat. No. 3,849,735 issued Nov. 19, 1974 to Haenen et al. and assigned to U.S. Philips Corporation.
Of importance, such prior art wide bandwidth amplifiers used to amplify video signals are probably operated as class A amplifiers, in order to achieve fast pull-down and pull-up of the output voltage in response to changes in the input voltage, without the added cost and circuitry required to drive the amplifier in other than class A and to stabilize the output signal. Unfortunately, as is well known, class A amplifiers require a relatively large amount of power. Prior art class A video driver circuits configured as integrated circuits typically consume at least 2 watts of power for a black and white video image, or 2 watts per each of the three primary colors in a color video system.
Offenlegungsschrift 26 53 624 describes a prior art class AB amplifier used to amplify video signals in which the NPN pull-down output transistor is driven by a voltage applied to its base, and the NPN pull-up output transistor is connected as a totem pole, as is also described on page 884 of "Amplifying Devices and Low-Pass Amplifier Design", John Wiley & Sons, 1968. The disadvantages of this well known prior art circuit are the difficulty in interfacing it to a low voltage integrated circuit driver and the inferior frequency response which results from driving the base of the output transistors.
Of importance, such prior art wide bandwidth amplifiers used to amplify video signals are probably operated as class A amplifiers, in order to achieve fast pull-down and pull-up of the output voltage in response to changes in the input voltage, without the added cost and circuitry required to drive the amplifier in other than class A and to stabilize the output signal. Unfortunately, as is well known, class A amplifiers require a relatively large amount of power. Prior art class A video driver circuits configured as integrated circuits typically consume at least 2 watts of power for a black and white video image, or 2 watts per each of the three primary colors in a color video system.
Class B amplifiers have also been used to amplify video signals in the prior art. U.S. Pat. No. 4,114,109 issued to Campioni describes such a class B amplifier circuit. Unfortunately, the '109 circuit is rather complex and requires the use of a significant number of relatively large value capacitors, thereby precluding its fabrication as an integrated circuit. Furthermore, the '109 circuit drives discrete PNP and NPN output transistors by providing appropriate signals to their bases. By driving the bases of the output transistors, very high frequency output transistors must be used since the base capacitance of a transistor which is driven by a signal applied to its base adversely affects the usable frequency response of that transistor.